The conventional document management device, e.g. the one disclosed in TW-200500899 (equivalent to US 2004/0267557, CN 1567326), is capable of saving an electronic document transmitted from a user end to a data folder corresponding to an address specified in the electronic document. However, when searching an electronic document saved according to this method, one can only try to locate the data folder by memory and then sequentially search through the files saved in the data folder to find out a desired electronic document. This causes users a lot of troubles. The present invention uses techniques, such as an optical identification device, a feature mark identifier, etc., to automatically set up a feature mark index at the time of saving files so that, while in later use, a user only needs to key-in one or more feature mark(s) of the electronic document in order to find the electronic document promptly.